This invention relates to a process for slow softening water as distinguished from water softening which takes place under pressure inside a reactor. As is well known in the art, slow softening of water is a method for removing the temporary hardness of water by admixing the water with an alkaline compound, usually Ca(OH).sub.2 or possibly NaOH or Na.sub.2 CO.sub.3, passing the water through a reaction zone wherein the various calcium carbonates present in the water, in particular soluble Ca(HCO.sub.3), are changed into insoluble CaCO.sub.3, contacting such compounds with a flocculating or precipitating agent containing trivalent metal ions, such as FeCl.sub.3, and settling the flocs thus produced in a sedimentation zone or separating such flocs by means of a sand filter.
A very difficult problem encountered with the known slow softening process lies in the fact that the reaction must be completed before the liquid leaves the reaction zone. That is, the total amount of calcium which can be precipitated must be changed into calcium carbonate so as to be completely flocculated and precipitated in the following stages. Otherwise, precipitation and scaling will occur inside pipes, ducts, heaters or other plant equipment through which the liquid subsequently passes. In known slow softening systems satisfactory reaction will occur only if the temperature of the liquid to be softened is greatly increased or if, for precipitation of calcium carbonate from supersaturated solutions, the aforementioned introduction of salts containing trivalent metal ions is combined with a retention time of several hours inside the flocculation and sedimentation zones. But even under these conditions and even if a series of successive flow-through, completely mixed reaction stages are used, it may occur, especially at temperatures below 5.degree.C., that reaction is incomplete and the calcium carbonate remaining in solution will later precipitate and scale in subsequent systems, pipes and ducts.